Purification of distilled coke-oven benzene by distillation with added hydrocarbon material



Jan. 81952 .1. R. ANDERSON PURIFICATION OF DISTILLED COKE-OVEN BENZENEBY DISTILLATION WITH ADDED HYDROCARBON MATERIAL Filed June 4, 1948INVENTOR. ./oH/v E, 4/vofkso/v. 8mm

Patented Jan. 8, 1952 PUBIFCATION F DISTILLED COKE-OVEN BENZENE BYDISTILLATION WITH ADDED IIYDROCARBON MATERIAL John IL Anderson,Pittsburgh, Pa., assignor to Koppers Company, Inc, Pittsburgh, Pa., acorporation of Delaware Application June 4. 194s, serali No. 31,098

(c1. 2oz- 42) 7 Claims. 1

This invention relates to the puriiication of aromatic hydrocarbons.More particularly this invention relates to a two-stage azeotropicdistillation method for separating pure aromatic hydrocarbons frommixtures that contain normally higher-boiling non-aromatic hydrocarbonswhich form azeotropes or non-ideal systems with the aromatic hydrocarboncomponent of the mixture.

It is well known that non-aromatic hydrocarbons may be separated fromaromatic hydrocarbons by one appropriate azeotropic distillation in aneiiicient rectifying column, in mixtures in which the non-aromatichydrocarbons normally have boiling points not signicantly higher thanthe aromatic components in the given mixture. For example, cyclohexane,a non-aromatic hydrocarbon normally boiling at 80.7 C., may be separatedfrom benzene, an aromatic hydrocarbon normally boiling at.80.l C., byazeotropicl distillation with acetone. Adequate azeotropic distillationmethods whereby non-aromatic hydrocarbons may be separated from mixturesin which the aromatic component or components have normal boiling pointssignificantly lower than the normal boiling points of the non-aromaticcomponents, however, have heretofore not been discovered. For example,benzene, an aromatic hydrocarbon normally boiling at 80.1 C., forms abinary azeotropemith normal heptane, a nonaromatic hydrocarbon normallyboiling at 98.4 C., and the separation of normal heptane from benzene byazeotropic distillation with, for ex ample, acetone, is a matter ofconsiderable difculty where the problem is that of obtaining purebenzene from such mixtures.

An important source of aromatic hydrocarbons is light oil ,which isobtained as a byproduct of the coking of coal. Benzene derived fromlight oil, by sulfuric acid-washing and emcient rectication, containsnon-aromatic hydrocarbons having normal boiling points and azeotropicproperties with benzene such that pure benzene cannot be obtained byeiiicient rectification. The principal impurities in coke oven benzenenormally boil above 90 C., and among the most troublesome impurities sofar as purification is concerned are normal heptane having a normalboiling point of 98.4 C., 2,2,4-trimethylpentane .having a normalboiling point of 99.3 C., and

methylcyclohexane having a normal boiling point of 100.9 C., whilebenzene has a normal boiling point of 80.1 C.

It has been found that benzene and the paraflins and naphthenes whichare usually associated with coke oven benzene form non-ideal and in mostcases azeotropic systems with ben,- zene. Further it has lbeen foundthat these nonideal or azeotropic systems boilvery close to the normalboiling point of benzeneand that they are composed almost entirely ofbenzene. Therefore, coke oven benzene which contains the normallyhigh-boiling impurities which form azeotropes with benzene cannot bepurified satisfactorily by ordinary rectiiication, no matter howefficient the rectifying equipment employed. In the most efcientfractionations, the overhead distillate will consist of an azeotrope ora mixture of azeotropes.

It has become a common practice to submit coke oven benzene toazeotropic distillation with polar azeotropic agents. A commonly usedpolar azeotropic agent for separating paraffin and naphthene impuritiesfrom coke oven benzene is acetone which is commonly known as a selectiveazeotropic agent in that it does not form an azeotrope with benzene. Ihave found, however, that acetone either does not form azeotropes withsome of the higher-boiling non-aromatic hydrocarbons present in cokeoven benzene or that if acetone forms such azeotropes their compositionsare such that no less than volumes of acetone are required to remove onevolume of the higherboiling contaminants. As a result, the purication ofcoke oven benzene by azeotropic distillation with acetone isunsatisfactory; the benzene remains somewhat impure, and the recovery ofacetone for recycling is an expensive operation because a considerableproportion of water must be added to the acetone distillate so that thesmall proportion of non-aromatic hydrocarbons can be made to separate asa second liquid phase.

Another commonly used polar azeotropic agent for the purication of cokeoven benzene is methanol. This alcohol forms azeotropes with al1 of thenon-aromatic hydrocarbon contaminants usually associated with coke ovenbenzene, but it is non-selective in its azeotropic behavior in that italso forms an azeotrope with benzene. Many tests have shown that thebenzene azeotrope of methanol interferes in the separation of thenon-aromatic hydrocarbons usually associated with coke oven benzene. Forexample, Where coke oven benzene, which contains normal heptane andmethylcyclohexane, is azeotropically distilled with methanol, normalheptane is concentrated somewhat` in the lower-boiling distillates vbutmethyl'cyclohexane is concentrated somewhat in the higher-boilingdistillates. In fact, both normal heptane and methylcyclohexane arefound in all of the distillates derived from such an azeotropicdistillation of coke oven benzene with methanol, presumably because ofthe closely related boiling points of the metliylcyclohexanemethanol,normal heptane-methanol and benzene-methanol azeotropes.

l Repeated tests have shown that the difllculties encountered in the useof methanol as an azeotropic agent for the purification of coke ovenbenzene are also found where employing other alcohols as azeotropicagents, for example ethanol, normal propanol, iso-propanol and butanol.The same difficulties are also encountered where employing methyl ethylketone as an azeotropic agent for the purification of coke oven benzene.Hence. methanol, ethanol, normal propanol, iso-propanol, butanol, anadmethyl ethyl ketone, which are water soluble polar azeotropic agents,are not suitable for removing the nonaromatic hydrocarbons which areusually associated with cokeV oven benzene.

Other aromatic hydrocarbons, such as toluene. ethylbenzene, orthoxylene,metaxylene, paraxyiene, and naphthalene, form azeotropes withnonaromatic hydrocarbons, and I have found that where these aromatichydrocarbons are associated with normally somewhat higher-boilingnonaromatic hydrocarbons, such as in aromatic iractions derived fromcoke oven light oil, they also cannot be separated from theirnon-aromatic contaminants by ordinary distillation or by azeotropicdistillation with polar azeotropic agents. Non-aromatic hydrocarbons andan aromatic hydrocarbon with a closely related boiling point formazeotropes with a common non-selective azeotropic agent which do notdiffer much in their azeotropic boiling temperatures. Hence the normalboiling point of the aromatic hydrocarbon cannot be substantially lowerthan the normal boiling points of the non-aromatic hydrocarboncontaminants in a given mixture without the non-aromatic hydrocarbonazeotropes having substantially the same azeotropic boiling temperatureas the aromatic hydrocarbon azeotrope of the given non-selectiveazeotropic agent. Where using selective azeotropic agents, thenonaromatic hydrocarbon contaminants cannot normally boil substantiallyabove the aromatic hydrocarbon without either the non-aromatichydrocarbon azeotropes failing to be formed or without theircompositions being 'exceedingly rich in the azeotropic agent.

I have discovered that aromatic hydrocarbons which contain significantlyhigher-boiling nonaromatic hydrocarbon contaminants which form non-idealor azeotropic systems with the aromatic component or components may bepurii fied by a method which consists ilrst in distllling the impurearomatic hydrocarbon so contaminated with a sufficient amount o! aselected nonaromatic hydrocarbon or mixture of non-aromatichydrocarbons. In that way, I can obtain aromatic-non-aromatic azeotropesor azeotropic l mixtures which do not contain non-aromatic hydrocarbonswhich normally boil signicantly above the normal boiling temperature ofthe aromatic hydrocarbon component, and after separation from thenon-aromatic hydrocarbon contaminants, these azeotropes may be distilledwith a polar azeotropic agent to obtain the pure aromatic hydrocarbons.

The primary object vof the present invention is to provide a method ofpurifying aromatic hydroearbons by a two-stage azeotropic distillationin which the first distillation will remove the aromatic hydrocarbonsoverhead from the non-aromatic hydrocarbons which boil significantlyabove the boiling'point of the aromatic hydrocarbons.

Another object of the invention is to provide a method of purifyingaromatic hydrocarbons by a two-stage azeotropic distillation in whichthe aromatic hydrocarbons are separated'from highboiling pointnon-aromatic hydrocarbons by distillation with a non-aromatichydrocarbon azeotropic agent and the non-aromatic hydrocarbon azeotropicagent used in the rst distillation is separated from the aromatichydrocarbon in a second distillation.

I prefer to use in the first distillation, a nonaromatic hydrocarbonwhich will form an azeotrope with the aromatic hydrocarbon in which thenon-aromatic hydrocarbon comprises from forty percent to sixty percentof the azeotrope. although non-aromatic hydrocarbons which formazeotropes with the aromatic hydrocarbon in which the non-aromatichydrocarbon comprises as little as ve per cent or as much as eighty percent of the azeotrope, or mixtures of nonaromatic hydrocarbons whichform azeotropes with the aromatic hydrocarbon in which the nonaromatichydrocarbons comprise from iive per cent to eighty per cent of theazeotropes may be used eiectively.

An essential condition in the first distillation is to employ from oneper cent to iive per cent more of the added non-aromatic hydrocarbon ormixture oi' non-aromatic hydrocarbons than is required to form theazeotrope or azeotropes with all of the aromatic hydrocarbon. Thisexcess of non-aromatic hydrocarbon ensures that substantially all of thearomatic hydrocarbon will be recovered and further it ensures thatsubstantially none of the normally higher-boiling contaminantsoriginally associated with the aromatic hydrocarbon will contaminate theoverhead distillate.

In the preferred form of my invention for separating pure benzene frommixtures that contain non-aromatic hydrocarbons which normally boilsubstantially above the normal boiling point of benzene but which whenpresent with benzene form azeotropes or non-ideal systems with benzene,I first distill the mixture with an amount of an added non-aromatichydrocarbon, which forms an azeotrope with benzene in which thenon-aromatic component comprises from forty per cent to sixty per centof the azeotrope which is taken overhead by distillation. Then I distillthe product of this distillation with an amount of added polarazeotropic agent which is sufiicient so that the non-aromatichydrocarbon, which was added in the iirst distillation and which nowmust be separated in a second distillation, is taken overhead bydistillation, vleaving pure benzene as the bottom product. The polarazeotropic agent forms an azeotrope with the non-aromatic hydrocarbon orhydrocarbons added in the first distillation, and the polar agentazeotrope of the non-aromatic hydrocarbon or hydrocarbons may beelectively resolved, for example by water extraction and decantation, sothat both the non-aromatic hydrocarbon or nonaromatic hydrocarbonmixture used in the first distillation and the polar azeotropic agentused in the second distillation may be recovered and recycled to theprocess.

Cyclohexane forms an azeotrope with benzene which contains approximatelyequal parts oi' benzene and cyclohexane. The boiling point ofcyclohexane is only sligthly higher than the boiling point of benzeneand the cyclohexane-benzene azeotrope is a distillate which can bereadily separated from the normally higher-boiling contaminantsassociated with coke oven benzene. Further, mixtures of cyclohexane andbenzene, such as the azeotropic distillate formed by cyclohexane andbenzene, may be readily separated by azeotropic distillation withacetone or with methanol. Still further, the cyclohexane-acetoneazeotrope or the cyclohexane-methanol azeotrope may be readily separatedby extraction and decantation with water. Therefore, ior thepurification of coke oven benzene, cyclohexane is a suitable hydrocarbonto be used in the first distillation and either acetone or methanol aresuitable polar azeotropic agents to be used in the second distillation.I have found, however, that any non-aromatic hydrocarbon or any mixtureof non-aromatic hydrocarbons boiling above 68. C. and below 90.5 C. canbe used effectively in the first distillation of the impure benzene. Thehydrocarbons boiling in this range include n-hexane, methylcyclopentane,2,2-dimethylpentane, 2,4-dimethylpentane, cyclohexane,2,2,3-trimethylbutane, 3,3-dimethylpentane and Z-methylhexane. Further,I have found that other aliphatic alcohols, such as ethanol andisopropanol, and another ketone, methyl ethyl ketone, may be usedeffectively in the second distillation.

Cyclohexane generally is not found in appreciable amounts in coke ovenbenzene but is present in petroleum distillates, in admixture with othernon-aromatic hydrocarbons, and is produced as a hydrogenation product ofbenzene.

A further object of the invention is to provide a two-stage distillationin which aromatic hydrocarbons are separated in the first distillationby a non-aromatic hydrocarbon or mixture of nonaromatic hydrocarbonswhich form azeotropes with the aromatic hydrocarbons in ubstantiallyequal parts, and thereafter separating the aromatic hydrocarbons fromthe non-aromatic hydrocarbon azeotropic agent by distillation with apolar azeotropic agent which forms an azeotrope with the non-aromatichydrocarbon so that the agent may be extracted from the non-aromatichydrocarbons with water and decantation.

Another object of the invention is to provide a method of purifyingaromatic hydrocarbons by atwo-stage azeotropic distillation in which thefirst stage azeotropic agent will form an azeotrope with the aromatichydrocarbon `which is cornposed of approximately equal parts of thedesired aromatic hydrocarbon and the non-aromatic hydrocarbon to permitthe desired hydrocarbon to be readily separated from the non-aromatichydrocarbon in a second distillation.

A further object of the invention is to provide a distillation processfor the separation of nonaromatic hydrocarbon contaminants which boilabove the boiling point of the desired aromatic hydrocarbon with anazeotropic agent which forms an azeotrope with the desired aromatichydrocarbon and using the azeotropic agent in sufcient excess to ensurethat the high boiling contaminants will not distill overhead with thedesired aromatic hydrocarbon.

Another object of the invention is to provide a method of separatingnon-aromatic hydrocarbons having a boiling point higher than benzenefrom benzene by a two-stage distillation using cyclohexane to azeotrope.with the benzene and carry the benzene-cyclohexane overhead in thefirst distillation and separating the benzene from the cyclohexane byazeotropic distillation with a polar azeotrope in the seconddistillation.

mately 105 C. to approximately 112 C. Pure toluene may be obtained fromthe mixture of toluene azeotropes obtained in the rst distillation byazeotropic distillation with polar azeotropic agents, for example,methyl ethyl ketone or methyl cyanide. As a general rule, I prefer thatthe mixture of non-aromatic hydrocarbons used as azeotropic agent in thefirst distillation have a boiling range of from not more than 5 C. belowto not more than 5 C. above the boiling temperature of the aromatichydrocarbon which is to be purified, although mixtures having some--what wider boiling ranges, for example, from 10 C. below to 5 C. abovethe boiling point of the aromatic hydrocarbon to be purified may be usedeffectively.

A still further object of the invention is to provide a two-stagedistillation process for purifying aromatic hydrocarbons having aboiling point higher than the boiling point of the aromatic hydrocarbonby an azeotropic distillation in the first stage using a mixture ofnonaromatic hydrocarbons having a boiling point not more than 5 above or5 below the boiling point of the desired aromatic hydrocarbon as theazeotropic agent for the first distillation and using a polar azeotropicagent for the second distillation.

With these and other objects and features in view. the inventionconsists in the improved method of purifying aromatic hydrocarbons ashereinafter described and particularly dened in the claims.

The various features of the invention are illustrated in theaccompanying drawing in which The figure is a diagrammatic flow diagramof an apparatus in which the preferred method of invention may becarried out while using a nonaromatic hydrocarbon azeotropic agent forthe first distillation, a polar azeotropic agent for the seconddistillation, and water for separating the aromatic hydrocarbon andnon-aromatic azeotropic agent from the distillation products.

The preferred form of the invention will be described more particularlywith reference to the purification of coke oven benzene although theprocess is well adapted to the purication of other aromatichydrocarbons, such as toluene, ethyl benzene, orthoxylene, metaxylene,paraxylene, and naphthalene.

Referring to the figure, impure benzene is taken from a storage tank l0and passed through line I2 into the mid-portion of a distilling columnI4. To the impure benzene entering the column is added cyclohexane fromstorage tank I6 through line I8. In the distillation in column I4 abenzene-cyclohexane azeotrope is taken overhead through a line 20 andintroduced into the mid-portion of a second distilling column 22. Theoverhead distillate from column I4 contains substantially all of thebenzene introduced into the column, and the impurities in the benzeneare removed from the base of the column through line 24, with a verysmall amount of cyclohexane therein. Cyclohexane containing a muchsmaller amount of benzene that is present in the cyclohexane-benzeneazeotrope is taken as a side cut from the lower part of column I4through line 2l and returned to feed line I2. The side cut which is richin cyclohexane ensures that no non-aromatic hydrocarbons normallyboiling significantly higher than cyclohexane will contaminate thecyclohexane-benzene azeotrope removed from column I4 through line 20.and that no benzene will be lost through line 24. To thebenzene-cyclohexane azeotrope is introduced through a line 26, a polarazeotropic agent such as methanol which forms an azeotrope with thecyclohexane as well as the benzene. or a polar azeotropic agent such asacetone which forms an azeotrope with the cyclohexane but not with thebenzene. The cyclohexane azeotrope of the polar azeotropic agent istaken overhead from column 22 through line 28 and the benzene along withany excess of polar azeotropic agent over that required to form thecyclohexane azeotrope is removed from the column 22 through a line 30.The cyclohexane azeotrope of the polar azeotropic agent is passedthrough line 28 into the midportion of a decanting separator 32 andwater is introduced into the separator through a line 34. A sufficientamount of water is introduced into the separator 32 to separate thecyclohexane from the polar azeotropic agent, the cyclohexane being thesupernatant liquid and passes through a line 35 back to the storage tanki6 for recycling through the column I4. If desired cyclohexane may bereturned directly to the line l2 by line 31 which is connected betweenlines I2 and 35. The solution of the polar azeotropic agent and waterows out of the bottom of the decanter 32 through a line 36 and isintroduced into the mid-portion of a distilling column 38. The polarazeotropic agent-benzene mixture flows from the bottom of column 22through the line 30 into the mid-portion of a decanting separator 40.Water is introduced into the separator 40 through a line 42 and thewater and polar azeotropic agent are separated by decantation in theseparator 40. The benzene is the supernatant liquid which passes througha dehydrator 43 and is recovered as pure benzene. The water-polarazeotropic agent mixture ows from the bottom of the separator 40 througha line 44 into the line 36 where it, along with the water-polarazeotropic agent mixture flowing from the bottom of separator 32 ispassed into the column 38. The mixture of water and the polar azeotropicagent is distilled in the column 38 to separate the polar azeotropicagent from water. The polar azeotropic agent passes overhead through aline 46 back to the line 20 and is recirculated through the seconddistillation column 22. Water flows from the bottom of column 38 througha line 48 to a water reservoir D. The reservoir 50 supplies water thatpasses through the line 34 and 42 to the separators or decanters 32 and40. If desired, water may be introduced into the reservoir 50 through aline 52.

In the continuous process illustrated in the drawing. azeotropic agents,such as cyclohexane and methanol, and water, are added to the mixturebeing continuously distilled and these agents are then recovered andreturned to their respective stages in the process. It is to beunderstood that a portion of these agents will be lost in the operationand, therefore, it is necessary to add to the various stages the agentsto maintain the desired volume of agents for carrying on the separationoperations.

The preferred form of the invention has been described with perticularreference to the purification of benzene where using cyclohexane as theazeotropic agent in the first distillation and methanol or acetone asthe azeotropic agent in the second distillation. It is to be understood,that other non-aromatic hydrocarbons such as 2,4-dimethylpentane,2,2-dimethylpentane and 2,2,3-trimethylbutane or mixtures ofnon-aromatic hydrocarbons having a boiling point range fromapproximately 75 C. to approximately C. may be used in the firstdistillation for the purification of benzene, and that other azeotropicagents which are polar and water-soluble, for example ethanol orisopropanol may be used in the second distillation. Further, it is to beunderstood that other aromatic hydrocarbons, for example, toluene,ethylbenzene, orthoxylene, metaxylene, paraxylene and naphthalene may bepurified by the method diagrammatically illustrated in the drawing butother appropriate nonaromatic hydrocarbons are to be used in the firstdistillation and appropriate polar azeotropic agents are to be used tothe second distillation. For example, in the purification of toluene,2,2,3- trimethylpentane or 3,3-dimethylhexane, or a mixture ofnon-aromatic hydrocarbons with a boiling point range of fromapproximately C. to 112 C. may be used in the iirst distillation, andmethyl ethyl ketone or methanol may be used in the second distillation.For other aromatic hydrocarbons, mixtures of non-aromatic hydrocarbonswith a boiling point range of not more than 10 C. below'and not morethan 5 C. above the boiling point of the aromatic hydrocarbon or anysingle non-aromatic hydrocarbon boiling within that range may be used inthe first distillation, and any polar and water-soluble azeotropic agentwhich will form a minimumboiling azeotrope with the agent added in thefirst distillation may be used in the second distillation.

The preferred form of the invention having been thus described, what isclaimed as new is:

I claim:

1. A method for the purification of distilled coke-oven benzene whichcontains a small amount of non-aromatic hydrocarbon impurities higherboiling than benzene and substantially inseparable therefrom by simpledistillation, which comprises distilling in a fractional distillationzone a mixture of said benzene and added nonaromatic hydrocarbonmaterial, said hydrocarbon material boiling normally between 75 and 85C. and being present in said mixture in an amount in excess of thatrequired to form an azeotrope with said benzene, removing from saiddistillation zone the azeotrope of benzene and said hydrocarbon materialas an overhead fraction, removing at least a portion of the excesshydrocarbon material and said high boiling impurities from saiddistillation zone as a distillation residue, separating benzene and saidhydrocarbon material by distillation with a polar azeotropic agent in asecond fractional distillation zone and recovering puried benzene freedof high boiling non-aromatic impurities from a fraction from said seconddistillation zone.

2. The method of claim l in which the polar azeotropic agent is acetone.

3. The method of claim 1 in which the polar azeotropic agent ismethanol.

4. A method for the purification of distilled coke-oven benzene whichcontains a small amount of non-aromatic hydrocarbon impurities higherboiling than benzene and substantially inseparable therefrom by simpledistillation, which comprises distilling in a fractional distillationzone a mixture of said benzene and added petroleum distillate boilingnormally between 75 and 85 C., said petroleum distillate being presentin said mixture and containing parans and naphthenes in an amount inexcess of that required to form azeotropes between the parains andnaphthenes and the benzene in said mixture, removing from saiddistillation zone said azeotropes as an overhead fraction, removing atleast a portion of the excess parains and naphthenes and said highboiling impurities from said distillation zone as a distillationresidue, separating benzene and said paratlins and naphthenes bydistillation with a polar azeotropic agent in a second fractionaldistillation zone, and recovering purified benzene freed of high boilingnonaromatic impurities from a fraction from said second distillationzone.

5. A method for the purification of distilled coke-oven benzene whichcontains a small amount of non-aromatic hydrocarbon impurities higherboiling than benzene and substantially inseparable therefrom by simpledistillation, which comprises distilling in a fractional distillationzone a mixture of said benzene and at least one added hydrocarbonselected from the group consisting of 2,2-dimethylpentane,2,4-dimethylpentane, cyclohexane, and 2,2,3-trimethylbutane, saidhydrocarbon being present in said mixture in an amount in excess of thatrequired to form an azeotrope with said benzene, removing from saiddistillation zone the azeotrope of benzene and said hydrocarbon as anoverhead fraction, removing at least a portion of the excess hydrocarbonand said high boiling impurities from said distillation zone as adistillation residue, separating benzene and said hydrocarbon bydistillation with a polar azeotropic agent in a second fractionaldistillation zone, and recovering purified benzene freed of high boilingnon-aromatic impurities from a fraction from said second distillationzone.

6. A method for the purification of distilled coke-oven benzene whichcontains la small amount of non-aromatic hydrocarbon impurities higherboiling than benzene and substantially inseparable therefrom by simpledistillation, which comprises distilling in a fractional distillationzone a mixture of said benzene and added cyclohexane, the cyclohexanebeing present in said mixture in an amount in excess of that required toform an azeotrope with said benzene, removing from said distillationzone the azeotrope of benzene and cyclohexane as an overhead fraction,removing at least a portion of the excess cyclohexane and said highboiling impurities from said distillation zone as a distillationresidue, separating benzene and cyclohexane in said azeotrope bydistillation with a polar azeotropic agent in a second fractionaldistillation zone, and recovering purified benzene freed of high boilingnon-aromatic impurities from a fraction from said second distillationzone.

7. A method for the purification of distilled coke-oven benzene whichcontains a small amount of non-aromatic hydrocarbon impurities higherboiling than said benzene and substantially inseparable therefrom bysimple distillation, which comprises distilling in a fractionaldistillation zone a mixture of said benzene and added cyclohexane, saidcyclohexane being present in said mixture in an amount in excess of thatrequired to form an azeotrope with benzene, removing from saiddistillation zone the azeotrope of benzene and cyclohexane as anoverhead fraction, removing at least a portion of the excess cyclohexaneand said high boiling impurities from said distillation zone as adistillation residue, separating benzene and cyclohexane by distillationwith acetone in a second fractional distillation zone, and recoveringpurified oenzene freed of high boiling non-aromatic impurities from thebottoms fraction from said second distillation zone.

JOHN R.. ANDERSON.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATS vPATENTS

1. A METHOD FOR THE PURIFICATION OF DISTILLED COKE-OVEN BENZENE WHICHCONTAINS A SMALL AMOUNT OF NON-AROMATIC HYDROCARBON IMPURITIES HIGHERBOILING THAN BENZENE AND SUBSTANTIALLY INSEPARABLE THEREFROM BY SIMPLEDISTILLATION, WHICH COMPRISES DISTILLING IN A FRACTIONAL DISTILLATIONZONE A MIXTURE OF SAID BENZENE AND ADDED NONAROMATIC HYDROCARBONMATERIAL, SAID HYDROCARBON MATERIAL BOILING NORMALLY BETWEEN 75* AND 85*C. AND BEING PRESENT IN SAID MIXTURE IN AN AMOUNT IN EXCESS OF THATREQUIRED TO FORM AN AXEOTROPE WITH SAID BENZENE, REMOVING FROM SAIDDISTILLATION ZONE THE AZETROPE OF BENZENE AND SAID HYDROCARBON MATERIALAS AN OVERHEAD FRACTION, REMOVING AT LEAST A PORTION OF THE EXCESSHYDROCARBON MATERIAL AND SAID HIGH BOILING IMPURITIES FROM SAIDDISTILLATION ZONE AS A DISTILLATION RESIDUE, SEPARATING BENZENE AND SAIDHYDROCARBON MATERIAL BY DISTILLATION WITH A POLAR AZEOTROPIC AGENT IN ASECOND FRACTIONAL DISTILLATION ZONE AND RECOVERING PURIFIED BENZENEFREED OF HIGH BOILING NON-AROMATIC IMPURITIES FROM A FRACTION FROM SAIDSECOND DISTILLATION ZONE.